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Which one of the following options is correct?

A)  Linear equation in two variables is in the form of \[ax+by+c=0,\] where \[a=0\]

B)  Linear equation in two variables is in the form \[px+qx+c=0\]

C)  Linear equation in two variables is in the form of \[sx+ty+r=0,\] where s, r and t are constants and \[s,\,\,t\ne 0\]

D)  Linear equation in three variables is\[ax+by+c=0,\] where a, b, c are constants and \[a,\,b\ne 0\]

E)  None of these